high pitched squeel`

ok i have a question about this crazy sounding noise i get whenever the rpms get to about the range of 5k to 6k. when my rpms get up there, there is a high pitch noise like eeeeerp. i dont know what it is. its killing me. the noise doesnt stay there it is like about a second long and the whole time the rpms are still going. what could it be. any help is appreciated.